What is a P&ID?
A Piping and Instrumentation Diagram (P&ID) is the master document of a process plant. It shows every piece of equipment, every pipe, every valve, and every instrument connected on a single set of drawings. Where the Process Flow Diagram (PFD) shows the overall process concept, the P&ID shows the plant as it will be built.
P&ID vs PFD: What Each Document Is For
A P&ID is frequently confused with a Process Flow Diagram (PFD). They serve different purposes and live at different points in the design lifecycle:
| PFD | P&ID | |
|---|---|---|
| Purpose | Overall process concept and heat / material balance | Definitive engineering record |
| Equipment | Major equipment only | Every item with tag and size |
| Piping | Main process lines only | Every line: size, service, spec, insulation |
| Instruments | Major control loops only | Every instrument and its loop function |
| Valves | Not shown | Every valve: isolation, control, safety, drain, vent |
| Drains / vents | Not shown | All drains, vents, sample points |
| Used for | Process design, heat balance | Operations, maintenance, safety, HAZOP |
| Status | Conceptual | Accurate to the installed plant |
Rule of thumb: never operate or safety-review from a PFD. The PFD answers "what does the process do?"; the P&ID answers "exactly how is it built?".
What Appears on a P&ID
- All process equipment with tag numbers and key data (T-101, P-201A/B, E-301)
- All piping with line numbers, sizes, and specification classes
- Every valve — hand, control, safety, isolation
- Every instrument, its measurement type, location, and control function
- Interlocks, alarms, and shutdown logic references
- Insulation, tracing, and slope requirements
Instrument Tag Numbering (ISA S5.1)
Instruments are identified by a function letter block and a loop number, e.g. FIC-201 or PSHH-315.
| First letter | Measured variable |
|---|---|
| F | Flow |
| L | Level |
| P | Pressure |
| T | Temperature |
| A | Analysis |
| H | Hand (manual) |
| J | Power |
| Succeeding letters | Function |
|---|---|
| I | Indicate |
| R | Record |
| C | Control |
| T | Transmit |
| S | Switch |
| V | Valve |
| E | Element (primary sensor) |
| Y | Compute / relay |
| H / L / HH / LL | High / Low / High-High / Low-Low |
Examples:
- TIC-101 = Temperature Indicating Controller, loop 101
- PSHH-201 = Pressure Switch, High-High, loop 201 (shutdown trip)
- FE-301 = Flow Element (primary sensor, e.g., orifice plate), loop 301
- LAH-402 = Level Alarm, High, loop 402
Line Numbering
Each pipe gets a line number of the form:
{size}-{service}-{sequence}-{spec}-{insulation}
For example: 6"-CWS-101-A1A-H = 6-inch diameter, chilled water supply, line 101, pipe spec A1A, hot-insulated.
The pipe spec (e.g., A1A, CS150, SS300) defines materials, wall thickness, flange rating, gaskets, and bolting for that line. All lines in the same spec share the same material of construction — this is fundamental to procurement and quality control. For sizing those lines once the service is fixed, see the Pipe Flow Engineering guide and the flow calculators in the related tools.
Common Symbols
| Symbol shape | Meaning |
|---|---|
| Bare circle | Field-mounted instrument |
| Circle with horizontal line | Panel-mounted (control room) |
| Circle with dashed line | Auxiliary panel |
| Hexagon | Computer/DCS function |
| Square with circle inside | PLC / logic function |
| Two triangles meeting | Gate or globe valve |
| Ball outline | Ball valve |
| Butterfly wafer | Butterfly valve |
| Bowtie with actuator top | Control valve |
Valve Symbols
Valves are the most symbol-dense area of a P&ID. It is worth separating isolation valves from control and safety valves — they play very different roles:
| Valve Type | Symbol Shape | Function on P&ID |
|---|---|---|
| Gate valve | Two opposing triangles (bow-tie) meeting at the line | Isolation/block; fully open or closed, not for throttling |
| Globe valve | Bow-tie with a small circle at the junction | Throttling or frequent isolation |
| Ball valve | Filled small circle (or circle with handle mark) | Quick quarter-turn isolation |
| Butterfly valve | Vertical line with two small arcs (wafer) | Isolation and throttling in large-diameter lines |
| Check valve | Open triangle pointing in the flow direction | One-way flow; prevents backflow |
| Diaphragm valve | Bow-tie with a horizontal bar over the body | Corrosive or slurry service with tight shutoff |
| Control valve | Bow-tie with actuator symbol (e.g., diaphragm/spring or motor on top) | Modulates flow, level, pressure or temperature automatically |
| Safety/relief valve | Distinct spring-loaded symbol venting to atmosphere or a header | Overpressure protection; opens at set pressure |
A quick reading habit: if the valve has an actuator symbol (FV, PV, LV, TV tags), it is a control device tied to an instrument loop; if it has no actuator and a hand-wheel mark, it is a manual isolation or throttling valve.
Piping Line Symbols
| Line Type | Drawing Convention | Meaning |
|---|---|---|
| Main process line | Heavy solid line | Primary flow path |
| Utility line | Medium solid line | Cooling water, steam, air, nitrogen services |
| Instrument impulse line | Thin solid line from process tap to transmitter | Sensing line for the instrument |
| Pneumatic signal | Dotted line with slashes | Air signal to a valve actuator or controller |
| Electrical signal | Dashed line | 4-20 mA, digital I/O or alarm wiring |
| Hydraulic / capillary | Zigzag or bubble line | Hydraulic signal or capillary fill |
| Insulated line | Solid line with an insulation note (e.g., H = hot insulated) | Thermal insulation required |
| Heat-traced line | Solid line with a tracing note (e.g., TR) | Steam/electric heat tracing |
| Jacketed line | Outer parallel line around the pipe | Double-wall construction |
| Sloped line | Line with an arrow and slope percentage | Gravity flow with a required fall |
| Spec break | A short vertical tick on the line | Pipe specification changes at this point |
Pump, Compressor and Motor Symbols
Rotating equipment carries its own symbol conventions. A pump is drawn as a circle with a triangle inlet joined to the discharge line and is tagged P-101; a duty/spare train appears as P-101 A/B. Compressors (C-), blowers (K-) and motors (M-) use similar conventions with the driver drawn beside or above the driven machine.
The symbol also shows the machine class: a centrifugal impeller, a gear / screw / lobe pump body, or a piston / plunger / diaphragm head, so a reader can tell the duty class at a glance. Choosing between a centrifugal and a positive displacement machine is driven by flow, viscosity, pressure and stability requirements — see the Positive Displacement Pumps guide for the comparison, and the Pump Power Calculator for the hydraulic duty each machine must satisfy.
Signal Line Conventions
- Solid line — process piping
- Dashed line — electrical signal
- Dotted line with slashes — pneumatic signal (typically /)
- Line with bubbles — capillary tubing
- Zigzag line — hydraulic signal
- Wavy line — sonic / radio
Common Instrument Loops
Most P&IDs use the same handful of loop configurations. Recognizing them quickly is the fastest way to read a drawing:
| Loop Tag | Measured Variable | Typical Configuration |
|---|---|---|
| FIC-xxx | Flow indicating control | FE (element) + FT (transmitter) + FIC (controller) + FV (valve) |
| PIC-xxx | Pressure indicating control | PT + PIC + PV |
| LIC-xxx | Level indicating control | LT + LIC + LV |
| TIC-xxx | Temperature indicating control | TE + TT + TIC + TV |
| PSHH-xxx | Pressure switch, high-high | Switch feeds shutdown logic (trip) |
| LAL-xxx | Level alarm, low | Alarm only, no control action |
| AIC-xxx | Analysis indicating control | AE + AT + AIC (e.g., pH, conductivity) |
When you see a control loop, ask three questions: what is measured (first letter), what is done with it (succeeding letters), and where is the final control element (the valve tag). That single sequence — sensor, transmitter, controller, valve — covers most automatic loops on a P&ID.
Worked Example: Reading a Simple Loop
Consider a control loop with these tags on the P&ID:
- FE-201 on the pipe (orifice element)
- FT-201 (differential pressure transmitter)
- FIC-201 in a circle with a horizontal line (controller in DCS)
- FV-201 (control valve on discharge)
Reading this loop: an orifice plate senses flow, the transmitter converts differential pressure to a 4-20 mA signal, the controller compares to setpoint and modulates the control valve. Together this is a flow control loop. If you see additional tags FSH-201 and FSL-201 connected, those are high and low flow alarms feeding the same loop.
Equipment Tags
| Prefix | Equipment |
|---|---|
| P | Pump |
| C | Compressor |
| E | Heat exchanger |
| T | Tank / vessel |
| V | Vessel (drum) |
| K | Blower |
| F | Filter |
| R | Reactor |
| S | Separator |
| M | Motor / mixer |
A duplicate equipment set is shown as P-101 A/B meaning two pumps, one operating and one spare.
Reading P&IDs Efficiently
- Start with the legend sheet. Every plant has one — it defines symbols and tag conventions for the site.
- Trace the process forward. Follow lines from feed to product, understanding the sequence of unit operations.
- Check every valve. Identify isolation, drain, vent, and bypass valves for each equipment item.
- Verify safety instrumentation. All PSHH, TSHH, LSHH tags represent trip functions — understand the shutdown logic.
- Confirm the line spec is consistent with the service. If cold service piping crosses a hot service via a heat exchanger, note the spec break.
Reading a Complete P&ID: Cooling Water Supply
Put the pieces together on a typical cooling water supply section (header line 8"-CWS-101-A1A-H feeding a heat exchanger):
- Pump station: Pumps P-101 A/B take suction from a basin through a strainer. Each discharge has a check valve (to prevent backflow through the idle pump) and an isolation gate valve before joining the common header.
- Flow control: FE-101 (orifice element) + FT-101 (transmitter) + FIC-101 (DCS controller) + FV-101 (control valve). The controller compares measured flow to setpoint and modulates FV-101.
- Pressure monitoring: PI-102 reads the header pressure, and PSHH-103 trips the pump on high-high pressure.
- Overpressure protection: PSV-104 on the header discharges back to the cooling tower basin.
- Line details: The 8"-CWS-101-A1A-H line is hot insulated (suffix H), spec A1A, with a spec break shown where the service changes near the exchanger.
A reliable reading sequence: start at the pump, trace flow through every valve and instrument to the exchanger, then list every trip and alarm (PSHH, TSHH, LAL) on the loop before checking isolation, drains, and spec breaks. That order — supply, control, protection, isolation — turns a busy drawing into four readable layers.
Best Practices for Engineers
- Always work from the latest revision — mark up P&IDs are legal documents.
- Use color highlighting when tracing a system for review or HAZOP.
- Cross-reference P&IDs to line lists, instrument index, and cause-and-effect matrices.
- When modifying a P&ID, follow Management of Change (MOC) — every red-line becomes an issued revision.
Summary
P&IDs are the definitive engineering record of a process plant. Fluency in reading them requires understanding ISA S5.1 tags, line numbering, and equipment symbols. Investing an hour learning your site's legend sheet pays dividends every day you work with the plant — for design reviews, safety studies, maintenance, and troubleshooting alike.
Frequently Asked Questions
What is the difference between a P&ID and a PFD? A Process Flow Diagram (PFD) shows the overall process concept with major equipment and heat/mass balance only. A P&ID shows every pipe, valve, drain, vent, and instrument as installed — it is the definitive engineering record used for operations, maintenance, and safety reviews.
What does FIC-201 mean on a P&ID? FIC-201 is an ISA S5.1 instrument tag: F = flow (measured variable), I = indicate, C = control, and 201 is the loop number. It identifies a flow indicating controller in loop 201.
What is the difference between a control valve and a safety valve on a P&ID? A control valve (e.g., FV-201) modulates flow continuously to maintain a process setpoint. A safety/relief valve is a protective device that opens automatically to relieve overpressure. Control valves are shown with an actuator symbol; safety valves have their own distinct symbol and are tagged with PSV/PSHH designations.
How do I read a line number like 6"-CWS-101-A1A-H? The line number encodes size (6"), service (CWS = chilled water supply), sequence number (101), pipe spec (A1A), and insulation (H = hot insulated). The pipe spec defines materials, wall thickness, flange rating, gaskets, and bolting for that line.
What is a loop number and why does it matter? A loop number groups all instruments serving one control function. Everything tagged 201 in a loop belongs to the same control loop — when troubleshooting, chase the whole loop by matching the number.
Related Engineering Tools
- Pressure Drop Calculator — Estimate line pressure loss for P&ID sizing
- Flow Rate Calculator — Convert flow and velocity for line sizing
- Pipe Velocity Calculator — Check design velocity in process lines
- Orifice Flow Calculator — Size orifice plates (FE elements)
- Control Valve Sizing Guide — Select control valves (FV tags)
- Process Plant Layout — Plot planning from the P&ID
- Valve Types and Applications — Common process valve types on P&IDs
- Positive Displacement Pumps — PD machine selection
- Pipe Flow Engineering — Line sizing and flow design for P&ID lines
- Safety Relief Valve Selection — Size relief valves (PSV tags)
- Chemical Plant Engineering — P&ID practice in process plants